Phylum: Mollusca

Mollusks are soft-bodied animals that usually have an internal or external shell.

The Mollusk Phylum has three Main classes:

1. Gastropods (ex. - snails)

2. Bivalves (ex. - clams)

3. Cephalopods (ex. – squid, octopus)

Class Bivalvia:

Bivalves have 2 shells held together by one or two powerful muscles.

Giant clam

Class: Gastropoda – Single shelled or shell-less and move with a muscular foot

Nudibranch

Sea hare

Snails

Slugs

Class Cephalopoda:Head is attached to a single foot. Foot is dividedInto tentacles or arms.

Octopus

Nautilus

Squid

Cuttlefish

Characteristics:

• Body has four parts: Foot, Mantle, Shell and Visceral mass

• Foot (Many forms): Used for crawling, burrowing and in the form of tentacles to capture prey.

• Mantle: Thin layer of tissue that covers the bodylike a cloak.

• Shell: Made by glands in the mantle that secrete calcium carbonate. In some groups the shell has been reduced or lost (slugs)

• Visceral mass – Internal organs

They can have an open or closed circulatory system.In an open system, the blood is pumped by a simple heart into the hemocoel.Hemocoel = interconnected sinuses/spaces within the mollusk’s body.

Specialized structures:

• Radula for feeding – tongue with tiny teeth.

• Siphon: (In aquatic species) A tube like structure through which water enters and leavesthe body.

• Nephridia to remove ammonia from blood; Help with excretion.• Clams and other bivalves – ganglia and nervecords, Octopi and squid – highly developed nervous system; they have well-developed brains.• Aquatic snails, clams – have gills in their mantlecavity. They are called ctenidia.

Mode of nutrition:

• They can be herbivores, carnivores, filter feeders or detritivores or parasites.• Octopi and some sea slugs have powerful jaws to eat their prey.• Some Octopi produce poisons to subdue their prey.

Mouth/Jawsof octopus

Movement:• Snails secrete mucus at the

base of the foot.• Octopus uses jet propulsion

Reproduction:

Reproduce in a variety of ways:• Some have external fertilization – eggs and sperm released in water. Fertilized eggs then develop intolarvae.• In others fertilization takes place inside the female.• Some are hermaphrodites. These usually fertilizeeggs from another individual.

Female octopus laying eggs,She lays about 57 000 eggs

Over a period of about 6 monthsShe grooms and protects the eggs

Octopi hatching. Mom helps by blowingwater currents across them to help them break free

Small octopus – mom usually dies;Usually only about 2 of the 57000 survive.